Studies were completed this calendar year which investigated the efficacy of growth hormone antagonists (GHA), developed by Sensus Corporation, in blocking the peripheral stimulation of the insulin-like growth factor (IGF) axis in adolescent male rhesus monkeys. Several formulations of the GHA at 0.1, 0.3 and 1.0 mg/kg, SC, were compared to animals receiving placebo. Animals were treated once weekly and serum samples were collected once weekly. Only the 1.0 mg/kg dose effectively suppressed serum concentrations of IGF-I and IGF binding protein-3 compared with placebo. These data suggest that the GHA may be clinically useful in blocking the action of GH in conditions of excessive GH secretion (acromegaly) or unopposed GH secretion (diabetes).